Foundations of permanent and mobile offshore structures have been undergoing evolution over the past five decades. The use of gravity base, pile, or mat foundations for platform structures in the early periods of offshore oil and gas drilling have clearly shifted to the deployment of caisson or spud-can type foundations. In the last two decades, the latter types are becoming more popular when mobile jack-up rigs are deployed for oil/gas drilling and production.
The foundation of an offshore structure is typically subjected to combined loads due to its self-weight and environmental forces; the environmental forces include lateral and over-turning loads created by wind, wave, and currents acting on the structures; seismic loads; and so on. To anticipate the expected maximum foundation loads that may occur during the structure's service life, the foundation is normally preloaded or proof-loaded to ensure that the foundation can provide the designed bearing capacities with an adequate safety margin. As is in practice, preloading is typically carried out by imposing static gravity loads, being equivalent to the anticipated maximum load, on the foundations and maintaining the loads for a certain period of time until no further settlement of the foundations occurs. Such a direct preloading method has been proven to be effective and is currently practiced.
Despite development of various established foundation types and proven preloading method, in some situations, existing foundation systems cannot offer technically and economically viable foundation solutions for mobile offshore structures. For example, mat foundation system is commonly used to support a mobile offshore platform installed on soft seabed by spreading the working load over a relatively large bearing or contact area. In spite of the resulting low bearing pressure under the foundation, the mat foundation still suffers from potential tilting, possibly due to load eccentricity and differential soil settlement, as well as potential horizontal instability occurring during service. In another example, the use of spud cans on soft seabed will typically lead to considerably deep leg penetration, which in turn results in difficulty in extracting the legs. When compressive load is predominant, conventional caisson foundations may not be an economically viable option with soft seabed because large diameter or deep-skirted caissons are required to provide sufficient foundation resistance.
The conventional preloading method also poses some potential risks, particularly when a mobile offshore platform supported by spud cans is preloaded on irregular seabed surface or punch-through prone areas; sliding or sudden penetration of any of the legs under preload can impose eccentric loads on the platform which in turn may create excessive bending moments on the legs and braces; this often leaves the rig operator with insufficient time to respond.
Apart from the potential risks associated with complexity of seabed conditions, the conventional preloading method also has some fundamental limitations. For example, foundation preloading for jack-up rigs is typically achieved by pumping sea water into ballast tanks located in the rigs' hull. These ballast tanks contributed to additional deadweights to the rigs which translates to additional loads on each leg. However, when large loads are required to ‘proof-test’ foundations located in harsh offshore environments or deep waters, increasing the volume of ballast tanks to meet the proof load requirements may not necessarily be always acceptable. On the other hand, for other types of independent offshore structures, such as minimum platforms or subsea storage tanks, providing ballast tanks solely for preloading purposes may not be an economical solution; providing for external ballast tanks may not be economical either.
Thus far, mat foundation system is normally adopted for mobile offshore platforms operating particularly in soft seabed. Some mat foundations have been modified to overcome somewhat problematic seabed conditions or operational constraints. For example, U.S. Pat. No. 4,668,127, issued to Bethlehem Steel Corp, uses pivotable mat-platform leg connections and multiple spuds extending downwardly on the mat base to tackle sloping seabed problem and lateral stability issue. In another example, U.S. Pat. No. 7,001,108, issued to Purvis, et al., provides a rig with a mat that has a central opening to overcome difficulties associated with the need for ballasting/deballasting the mat's compartments. The mat has buoyancy that supports the hull and legs in a floating position when the rig is in transit, whilst allowing the mat to be lowered to the sea bottom without assistance of ballasting/de-ballasting pumps. In another example, U.S. Pat. No. 4,265,568, assigned to The Offshore Company, uses spaced-apart reaction members to spread load to the seabed, instead of using a large single mat or gravity base, in attempts to overcome foundation rotational stability issues. The reaction members are allowed to penetrate into the seabed so that a bottom of the supported structure is not in contact with the seabed. In soft seabed condition, however, relatively massive reaction members may be required to resist the design vertical load.
The relatively large area of a mat or reaction members required to provide stability for its deployment on soft seabed remains an unresolved main drawback of such foundation systems. In addition, the conventional preloading method, i.e. by adding ballasts in the mat's compartments and platform's hull may cause uneven bearing pressure or differential settlement due to potential lateral variability of the seabed. The dependency on ballast tank to facilitate preloading may also hinder known foundation systems from being deployed in deeper waters or when ballasting method is not technically viable due to specific constraints such as that associated with total weight or stability of the structure.
It can thus be seen that there exists a need for a new foundation system that is viable for any seabed condition, in particular soft seabed, yet offering ease in installation/removal and stability during operation. With the industry shift to the use of mobile structures for offshore oil/gas drilling and production in harsher environments and deeper waters, there also exists a need to equip the new foundation system with integral elements which would enable preloading without reliance on ballast tanks and facilitating removal of the foundation.